1. Technical Field
The invention refers to a porous material for catalytic conversion of exhaust gases. The porous material includes a carrier with a first porous structure, and an oxidation catalyst which in the presence of oxygen, according to a first reaction, has the ability to catalyze oxidation of nitrogen monoxide into nitrogen dioxide and, according to a second reaction, to catalyze oxidation of a reducing agent, which oxidation catalyst is enclosed inside the first porous structure.
Preferably, the porous material also comprises a carrier with a second porous structure and a reduction catalyst that, in the presence of the reducing agent, is able to further catalyze reduction of nitrogen dioxide into nitrogen, whereby the reducing agent is at least partially consumed. The invention also relates to a method and arrangement for a catalytic conversion device that advantageously utilizes the porous material.
The invention may be applied within the field of catalytic conversion of exhaust gases that originate from internal combustion engines, particularly Lean Combustion engines (LC engines) and diesel engines.
The present invention may also be utilized for other exhaust gases that contain nitrogen oxides and that have an oxygen surplus, and which originate from stationary emission sources such as gas turbines, power plants and similar facilities.
2. Background Art
When attempting to reduce the emissions of nitrogen oxides (NOX) from internal combustion engines, great effort has been taken to modify the combustion conditions in order to reduce the NOx-emissions, while still maintaining the combustion efficiency at a satisfactory level.
Traditional techniques for the reduction of NOx-emissions include Exhaust Gas Recirculation (EGR), as well as special designs for fuel injectors and combustion chambers. Other important engine parameters are compression, fuel injection time and fuel injection pressure. Techniques involving water injection, the use of fuel/water emulsions and so-called Selective Catalytic Reduction (SCR) by ammonia have also been employed. From these techniques, it has been found that one-sided optimizations of combustion efficiency often results in increased NOx emissions.
Today it is required that both fuel consumption and NOx emissions be minimized. There are also strong demands for reducing emissions of other chemical compounds that are potentially hazardous to the environment such as hydrocarbons.
Accordingly, there is an increased need for catalytic converters that also are able to treat exhaust gases from the so-called Lean Combustion (LC) engines. Therefore, a number of different catalytic converters have been developed and are well known in commercial applications such as motor vehicles.
Typically, conventional catalytic converters include one or several matrices, or monolith bricks as they sometimes referred. Such bricks or monoliths typically take the form of a ceramic honeycomb substrate, with through passages or cells, and can be furnished with a porous surface coating. Particles of a suitable catalyst are embedded in the surface of the matrix, and the design of the matrix is optimized to maximize the surface area over which catalytic reactions take place. Common catalysts are noble metals such as silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), gallium (Ga), ruthenium (Ru) and mixtures thereof. There are also a number of other metals and metal oxides that can be used as catalysts. Such catalysts may have the ability to catalyze oxidation, affect reduction reactions, or both.
It is also known to use crystalline aluminum silicates, so-called zeolites, that are loaded with a suitable catalyst. The use of zeolites in connection with the catalytic conversion of exhaust gases are exemplarily disclosed in EP 0 499 931 and EP 0 446 408.
Furthermore, it is known to combine several different catalytic matrices, or to arrange a so-called after-burner in the catalytic conversion process. Such arrangements are, for example, disclosed in U.S. Pat. No. 5,465,574.
It is also previously known to use a honeycomb monolith of corrugated metal foil coated with a suitable catalytic material carried or supported on its surface.
It has also been suggested, for example in EP 0 483 708 A1, to combine a conventional ceramic catalytic converter with an electrically heatable catalytic converter in order to ensure that the optimum temperature for catalytic conversion is maintained.
From these examples, it can be appreciated that a number of different catalyst materials, devices, and arrangements utilized in catalytic conversion of exhaust gases are known.
From this, it would seem that simultaneous elimination of nitrogen oxides (NOx) and hydrocarbons (HxCx) could take place over, for example, an Ag-catalyst, according to the simplified chemical reactions:A) NOx+HxCy−>N2+H2O+CO2+CDandB) O2+HxCy−>H2O+CO2
In practice, however, it has been found that the following reaction predominates:C) NO2+HxCy−>N2+H2O+CO2
It should be noted that the term HxCy in the chemical reactions cited herein not only refers to hydrocarbons, but can also signify other reducing agents that include oxygen and/or sulphur. Accordingly, the reducing agent HxCy could also be expressed as HxCyOzSw. Examples of reducing agents that might be present in exhaust gases are alkanes, alkenes, paraffines, alcohols, aidehydes, ketones, ethers or esters and different sulphur-containing compounds. Also CO and H2 can act as reducing agents. Reducing agents found in exhaust gases can originate from the fuel or the combustion air, or it can be purposefully added to the exhaust gases.
It has earlier been found that the above-mentioned reaction according to C) is very rapid over, for example, Ag-catalysts. Acidic catalysts (H+) and acidic zeolites, doped with Ag or other suitable catalysts, have been found to be selective in the sense that NO2 will readily be converted, whereas NO will not. This can be of great disadvantage since NO is predominant in “lean” exhaust gases produced, for example, from LC-engines. Another problem is that the available amount of NO2 can become limiting for the reduction of hydrocarbons (HxCy) or other undesired compounds. The doping of zeolites with silver (Ag) or other metals such as iron (Fe) is exemplarily disclosed in EP 0955080 and EP 0406474 which disclose low silica MFI as a good basic structure for use in the mentioned applications.
In order to solve this problem; that is to be able to reduce the amount of both NO and HxCy in the exhaust gases, it has been suggested to combine an Ag-zeolite catalyst with a Pt-catalyst.
Normally, the following main reactions will take place over a conventional Pt-catalyst:D) NO+½O2+NO2E) O2+HxCy−>H2O+CO2F) 2NO+HxCy−>N2O+H2O+CO2
When using a conventional Ag-zeolite catalyst in combination with a conventional Pt-catalyst, all four reactions C), D), E) and F) will occur. However, since hydrocarbon (HxCy) is consumed in the chemical reactions E) and F), there is a risk that there will not be a sufficient amount of hydrocarbon (HxCy) left for the reaction with nitrogen dioxide (NO2), according to reaction C). This results in an undesired residue of nitrogen dioxide (NO2) in the catalytically converted exhaust gases, originating from reaction D).
Previous attempts have been made to solve this problem with different types of catalysts by such means as combining different catalysts, and by means of adding an additional amount of hydrocarbon to the exhaust gases in order to supply the reaction C) with a sufficient amount of hydrocarbon.
Many of the previous solutions, however, have been associated with the problem of undesired oxidation of hydrocarbons (HxCy) over at least some surfaces of the oxidation catalyst, which should preferably only catalyze oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2) according to reaction D).
Another problem associated with many previously known catalysts is that, during certain conditions, they catalyze according to reaction F) which produces dinitrogen oxide (N2O). This reaction is undesired and it is preferred that the nitrogen oxides (NOx) in the exhaust gases be converted into nitrogen (N2) to the highest possible degree, and not into dinitrogen oxide (N2O).
In WO 99/29400 it has been proposed to solve this problem by providing a porous material for catalytic conversion of exhaust gases, by which it is possible to selectively catalyze the oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2) and avoid catalytic oxidation of hydrocarbons (HxCy) or other reducing agents.
This is achieved by means of a porous material used for catalytic conversion of exhaust gases that include a carrier with a first porous structure, and an oxidation catalyst. In the presence of oxygen, the oxidation catalyst has the ability to catalyze oxidation of nitrogen monoxide into nitrogen dioxide, according to a first reaction. Furthermore, the oxidation catalyst, in itself, has the ability to catalyze oxidation of a reducing agent, according to a second reaction. According to WO 99/29400, the oxidation catalyst is enclosed inside the first porous structure, which first porous structure has pores with such dimensions that the reducing agent is sterically prevented from coming into contact with the oxidation catalyst. This will enable primarily the first reaction, out of said first and second reactions, to take place over the oxidation catalyst during the catalytic conversion of the exhaust gases.
According to WO 99/29400, the catalyst further includes a reduction catalyst enclosed in a second porous structure that has pores with greater dimensions than the pores in the first porous structure. This enables the reducing agent to react with the nitrogen dioxide (NO2) according to a third reaction.
Using the described porous structures of two different sizes, as is described above, an improvement for a selective catalytic reaction is provided. There is, however, still a need for even better applications and methods for catalytic conversion of exhaust gases. For example, the porous material described uses strong oxidation agents, preferably such as platinum (Pt) and/or Palladium (Pd), and this puts high demands on the production. It is important that such strong oxidation agents do not contaminate the outside of the oxidation pore, since such a contamination would oxidate and consume some of the HxCy, thereby diminishing the reducing agent necessary for reducing NO2to N2 according to the favorable third reaction.
Accordingly, a need has been recognized for an improved, selective oxidation catalyst material, that catalyzes oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2) and which does not catalyze oxidation of hydrocarbons.
Furthermore, there is also a recognized need for an effective combination of such a selective oxidation catalyst material for catalyzing a reaction that produces nitrogen dioxide (NO2) and a reduction catalyst material for catalyzing a reaction in which nitrogen dioxide (NO2) is reduced by hydrocarbons or other reducing agents into nitrogen (N2).